1. Field of the Invention.
The present invention relates to the field of electrostatic powder spray coating systems.
2. Prior Art.
For many manufactured goods it is necessary to paint one or more surfaces thereof so as to provide a decorative and/or protective coating to the surface. Conventional paints however, are not easy to apply on a production basis as lack of accurate control of the coating thickness may result in runs or inadequate coating of the article being painted. This is particularly true of parts having complex shapes, which encourage uneven coating thicknesses. Further, the commonly used paint solvents necessary to allow proper spraying of the paint are expensive and do not add to the coating, present a fire hazard, and particularly, in recent years, are highly restricted because of environmental considerations. Further, solvent base paints generally require some time at temperature for the proper curing thereof, a definite manufacturing disadvantage. Even if the time element could be eliminated, the efficiency of the spray painting process is less than optimum because of the substantial over-spray often encountered, depending in part on the experience of the painter and the shape of the part being painted.
Processes for coating parts with a solid resin coating provide other decorative and/or protective coating techniques, and have found ever increasing application in recent years because of their many advantages over ordinary solvent base paints. In accordance with these processes, the resin coating is prepared in a powder form and sprayed on the parts to be coated much like a liquid paint. The parts however, are generally connected to one electrode of a direct current power supply, with an electrode of opposite polarity being provided at the orifice of the spray gun so that the part to be coated and the spray are given opposite charges. This results in electrostatic attraction between the powder and the parts to attract the powder to the surface of the part and keep it temporarily clinging thereto. Typically, as the powder builds up on the part, the electric charge on the part is offset by the opposite charge of the powder so that when a powder coating of the desired thickness is achieved, the electrostatic attraction is reduced to zero, thereby allowing additional powder to merely fall off the part. This self-limiting characteristic of the powder makes it much easier to cover hard to get at areas without overcoating adjacent areas. Since the resin will melt at an elevated temperature to provide the desired coating, such processes generally have the advantage of not requiring substantial time at temperature, thereby minimizing the "curing" equipment requirements.
The resin powder (hereinafter referred to simply as "powder") is generally sprayed in a powder spray booth provided for this purpose, with the parts to be coated being hung on a conveyor so as to pass through the spray booth at an appropriate speed. Such spray booths are generally provided with some form of collection or filtering system, whereby the overspray and excess powder falling off of the parts may be collected for reuse and/or to avoid environmental contamination. For this purpose, an air flow system is generally provided whereby air (and powder) are withdrawn from the spray booth area into a plenum wherein much of the powder will settle out. The remaining powder is generally filtered out of the air stream by appropriate filters provided for this purpose. Generally, the powder that settles out of the air is recycled, depending upon the specific design of the equipment.
Various types of filters have been used in the prior art. By way of example, bag type filters have been commonly used in such equipment, such filters generally comprising long, small diameter filter bags over appropriate filter tubes, with an appropriate fan withdrawing air from the center of the filter tubes so as to cause a flow generally directed inward through the filter tubes so that the dust filtered out collects on the outer surface of the tubes. It is also known in such equipment to occasionally provide a pulse of air in a reverse flow direction through the filters to essentially blow the filters clean, whereby much of the powder blown therefrom will settle out for recycling. Generally speaking, fluidized beds may be used to fluidize the powder which has settled out for recycling. Such fluidized beds generally have a porous floor member, which allows the injection of air into the powder to essentially expand the powder and allow the particles thereof the flow over each other, thereby resulting in flow characteristics much like ordinary fluids.
The foregoing equipment performs well in a ordinary production environment wherein only a single color is to be applied, or at least in situations wherein color changes are at most infrequent. However, in instances wherein frequent color changes are required, substantial down time may result. In particular, at least in the type of equipment utilizing periodic blow down to clean the filter bags, the filter bags cannot be cleaned well enough to allow the change of color without changing the filter bags also. This, coupled with the fact that prior art equipment of this type normally has the filter, mounting structure, etc. fastened to the spray booth itself, makes color changing a messy and time-consuming operation. In the inventor's earlier U.S. Pat. No. 4,277,260, powder collectors are disclosed which, in substantial part, overcome some of these problems by utilizing cartridge filters having a blow down capability and a detachable powder cart to more readily facilitate the change of powder colors by the change of the carts. The present invention however, represents an improvement in the powder collectors of the earlier patent to even more readily facilitate powder color changes to provide a more flexible spray booth and to provide for transportability of the entire system to different positions within a manufacturing plant.